1. Field of the Invention
The present invention is concerned with a method of catalyzing the reduction of nitrogen oxides with ammonia, especially the selective reduction of nitrogen oxides, with ammonia in the presence of oxygen, using zeolite catalysts, especially metal-promoted zeolite catalysts. The invention is also directed to hydrothermally stable zeolite catalysts and methods of making same.
2. The Related Art
Both synthetic and natural zeolites and their use in promoting certain reactions, including the selective reduction of nitrogen oxides with ammonia in the presence of oxygen, are well known in the art. Zeolites are aluminosilicate crystalline materials having rather uniform pore sizes which, depending upon the type of zeolite and the type and amount of cations included in the zeolite lattice, range from about 3 to 10 Angstroms in diameter.
Japanese Patent Publication (Kokai) No. 51-69476, published Jun. 16, 1976 on Application No. 49-142463, filed Dec. 13, 1974, discloses a method for reducing nitrogen oxides in waste gases by reaction with ammonia in the presence of a metal-promoted, dealuminized synthetic or natural mordenite zeolite. The resistance of the catalyst to sulfurous poisons, particularly sulfur trioxide and sulfuric acid mist, is said to be enhanced by dealuminizing the mordenite to increase the silica to alumina ratio to more than 12, preferably to more than 15. The zeolite is promoted with 0.5 to 30 wt. % of at least one of a number of promoters including copper, vanadium, chromium, iron, cobalt or nickel and used at a reaction temperature of 200° C. to 500° C. with from 0.5 to three times the stiochiometric amount of ammonia reductant. Example 1 of the Publication illustrates an iron-promoted mordenite ore as being effective for the reduction of nitrogen oxides. In connection with Example 2, it is stated that a slight decrease of the activity of a high silica to alumina ratio, copper-promoted mordenite catalyst is recognized when sulfur trioxide is included in the gas stream. However, an “extreme improvement” of resistance to sulfur trioxide poisoning is noted in comparison with a copper mordenite which has not been dealuminized to increase the silica to alumina ratio.
UK Patent Application No. 2,193,655A discloses a catalyst containing a low surface area titania and a copper-promoted zeolite for use in the reduction of nitrogen oxides with ammonia. The zeolite has an average pore diameter of 10 Angstroms or less, preferably 8 Angstroms or less, and a silica to alumina molar ratio of 10 or more, preferably 20 or more; the resultant titania/-promoted zeolite catalysts having these characteristics are stated to have good mechanical strength and to be resistant to volatile catalyst poisons such as arsenic, selenium, tellurium, etc., contained in exhaust gases. Examples of suitable zeolites are mordenite, ZSM-5, and ferrierite.
U.S. Pat. No. 4,297,328 discloses a “three-way conversion” catalytic process for the simultaneous catalytic oxidation of carbon monoxide and hydrocarbons and reduction of nitrogen oxides for purifying the exhaust gas of automobile engines operated within a prescribed range of air to fuel ratio (column 4, lines 63-68). The disclosed catalyst is a copper-promoted zeolite having a silica to alumina ratio greater than 10, preferably greater than 20 (column 6, lines 23-28). Representative high-silica zeolites are described at columns 6-8 of the patent and include (column 6, lines 29-33) silicalite (as described in U.S. Pat. No. 4,061,724), ZSM-8, ZSM-11, ZSM-12, hyper Y, ultrastabilized Y, Beta, mordenite and erionite. Ultrastabilized Y is described (column 7, lines 22-25) as “a form of zeolite Y which has been treated to give it the organophilic characteristic of the adsorbents of the present invention.” Example 6 of the patent is stated to show no measurable loss in combustion activity of the copper-promoted zeolite catalyst due to sulfur poisoning (exposure of the catalyst to methylmercaptan in the gaseous stream). The patent thus discloses the utility of the copper-promoted specified zeolites for three-way conversion in an exhaust gas generated by a lean air to fuel ratio combustion mixture.
The art thus shows an awareness of the utility of metal-promoted zeolite catalysts including, among others, iron-promoted and copper-promoted zeolite catalysts, for the selective catalytic reduction of nitrogen oxides with ammonia.
In accordance with U.S. Pat. No. 4,961,917, there is provided an improved method for the reduction of nitrogen oxides with ammonia. The method described in this commonly assigned U.S. patent comprises the following steps. A gaseous stream containing nitrogen oxides and ammonia, and which may also contain oxygen, is contacted at a temperature of from about 250° C. to 600° C. with a sulfur-tolerant catalyst composition. The catalyst composition comprises a zeolite having a silica to alumina ratio of at least about 10, and a pore structure which is interconnected in all three crystallographic dimensions by pores having an average kinetic pore diameter of at least about 7 Angstroms, e.g. from about 7 to 8 Angstroms, and one or both of an iron and a copper promoter present in the zeolite, for example, in the amount of from about 0.1 to 30 percent by weight, preferably from about 1 to 5 percent by weight, of the total weight of promoter plus zeolite. The zeolite comprises one or more of USY, Beta and ZSM-20. A refractory binder may be admixed with the zeolites. An iron-promoted zeolite beta is preferred and has been commercialized for removing NOx by selective catalytic reduction such as from gas turbine exhaust.
The iron-promoted zeolite beta has been an effective catalyst for the selective reduction of nitrogen oxides such as by the reduction of nitrogen oxides with ammonia. Unfortunately, it has been found that under harsh hydrothermal conditions, such as reduction of NOx from gas turbine exhaust at temperatures exceeding 500° C., the activity of the iron-promoted zeolite beta begins to decline. This decline in activity is believed to be due to destabilization of the zeolite such as by dealumination and consequent reduction of metal-containing catalytic sites within the zeolite. To maintain the overall activity of NOx reduction, increased levels of the iron-promoted zeolite catalyst must be provided. As the levels of the zeolite catalyst increase so as to provide adequate NOx removal, there is an obvious reduction in the cost efficiency of the process for NOx removal as the costs of the catalyst rise.
In copending, commonly assigned U.S. patent application Ser. No. 09/712,210, filed Nov. 15, 2000, improvements in the hydrothermal stability of aluminosilicate zeolite NOx reduction catalysts are described. In particular, stabilized aluminosilicate zeolites are provided by incorporating into the zeolite structure extra-framework aluminum oxide chains, which are believed to be associated with or even linked to the aluminosilicate framework of the zeolite. The presence of the extra-framework aluminum oxide chains is manifest by a unique peak found in the FT-IR spectrum. The presence of this peak at 3781±5 cm−1 is associated with the improved stability of the zeolite. The extra-framework aluminum oxide chains can be incorporated into the zeolite structure by several processes known at this time, including via a unique steaming regimen or by treatment with rare earth metals, such as cerium. While not wishing to be bound by any theory, it is believed that the treatment of the aluminosilicate zeolite decouples aluminum oxide temporarily from the zeolitic framework. The decoupled aluminum oxide molecules are then recombined and linked as a chain, which is reattached to or otherwise associated with the zeolite framework. The treatment process is unlike well-known methods of dealuminizing zeolites for the purpose of increasing the silica to alumina ratio. In the present invention, the alumina is not removed from the zeolite but is believed to be rearranged and otherwise attached or associated with the aluminosilicate framework. The extra-framework aluminum oxide chains associated with the FT-IR absorption peak at 3781±5 cm−1 appear to stabilize the zeolite against further dealumination such as under oxidizing and harsh hydrothermal conditions.
There is still a need, however, to improve the process for the selective catalytic reduction of NOx by ammonia so as to maintain catalytic activity, even under harsh hydrothermal conditions.
There is a further general need for improving the hydrothermal stability of aluminosilicate zeolite catalysts, especially metal-promoted zeolites so as to stabilize such materials from dealumination and loss of catalytic sites during use.